I rang Michelin tech support again and:
The bloke confirmed that there will be a slight increase in grip as the tyres wear from new peaking at around half worn.
He confirmed that as you approach the wear bars the tread wear reduces due to the tread not heating up as well.
He said that from about half worn the grip plateaus until the wear bars.
He said that in normal usuage (12k miles) he didn't think heat cycling would play a part in changing grip (making the tyres last say > 50000 miles might well introduce tyre compound hardening through heat cycling).
It seemed to me that increased tread life at near the wear bars should equate to less grip and I pressed him on this a couple of times but he was adament that there would be no significant decrease in grip from the peak at about half worn to the wear bars.
Don't know if this makes it any clearer.

And they have possibly two data points maximum - new-ish tire vs. shaved to a constant depth? It is not inconsistent to have a new partially shaved tire exhibit more traction than a nearly new tire and yet have an extremely worn one have way less traction. So the observations myself and others have made are not really entirely inconsistent with the observations in the book.

Again, see my point just above. I would be a fool to argue against shaving tires to increase grip for many reasons; it is a generally performed and successful endeavor, it has theory on its side and I have no doubt there is plenty of data to support the procedure. However, one point making that not entirely "apples to apples" is that I am talking about a UHP street tires and shaving is generally done on dedicated track tires.

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Let me propose a thought experiment. If you could take a new set of tires, break them in just enough to get rid of their low traction upon being brand new, then do an instrumented skid pad test on it. Have preped in a que 10-20 or so identical sets of tires but shave them say 1/64"-1/32" at a time, mount set each successively and skip pad test them. My guess is that the traction vs. amount shaved will be basically a bump, it will increase for a while, peak and then decrease. It will undoubtedly decrease at some point - say when you hit cords. The question is will is decrease substantially before that point and will it decrease to a point much lower than the nearly new set? I think they will, once near or significantly into the wear bar and at the point when the tread pattern really fundamentally changes I think you will have already lost substantial grip. This is precisely what I believe many of us have observed. Sure UV/aging/thermal cycling etc. may also be contributing through confounding effects. Nonetheless, I think you simply must find this bump shaped curve (before cording).

Ok here I go.I have experiance in this but it was quite a few years ago in the Firehawk series.We normally had our tires shaved at 4/32 and would get 3+ hours out of the highest loaded tires at most of the tracks that we would go with the tires getting to their quickest just before they started to cord and were totaly bald.For the times that qualifying was an issue because of large grids and people being sent home(Montreal Gp,Toronto Indy),the tires would be shaved to 2/32(minimum by the rules) and heat cycled once and saved for qualifying and would usually be a fair bit quicker than the 4/32 tire worn to 2/32 by use.We tried this on some test days to confirm this.Once we figured this out our tires were shaved to the minimum that we needed for the race length at the particular track.This was on spec tires that had a wear rating of 340 not a R-Compound tire by any means.

Ok here I go.I have experiance in this but it was quite a few years ago in the Firehawk series.We normally had our tires shaved at 4/32 and would get 3+ hours out of the highest loaded tires at most of the tracks that we would go with the tires getting to their quickest just before they started to cord and were totaly bald.For the times that qualifying was an issue because of large grids and people being sent home(Montreal Gp,Toronto Indy),the tires would be shaved to 2/32(minimum by the rules) and heat cycled once and saved for qualifying and would usually be a fair bit quicker than the 4/32 tire worn to 2/32 by use.We tried this on some test days to confirm this.Once we figured this out our tires were shaved to the minimum that we needed for the race length at the particular track.This was on spec tires that had a wear rating of 340 not a R-Compound tire by any means.

Interesting. Your experience is completely in line with the conclusion drawn in the Skip Barber book about the grip freshly shaved street tires will provide on the track at different shaved tread depths based on their testing. There is a question around if somewhat softer compound street tires such as the PS2 fall under that category as well, but they probably do. The info SenorFunkyPants was provided by the Michelin rep kind of suggests that they would.

Sounds interesting and promising but not entirely consistent. Can you really call it a "peak" in grip at about mid life when after that point the grip is maintained constant on a plateau? I would not call that "peak" per se, just a plateau. It also does sounds quite fishy that tread wear decreases but grip stays constant (again after about mid life). Lastly when does the plateau end does it simply hit a vertical "brick wall" right at the wear bar or is it right at cording? It must come down at some point and even get below the grip when just broken in. If it drops in the way he described then it must come down sharply, like going over a cliff.

Interesting. Your experience is completely in line with the conclusion drawn in the Skip Barber book about the grip freshly shaved street tires will provide on the track at different shaved tread depths based on their testing. There is a question around if somewhat softer compound street tires such as the PS2 fall under that category as well, but they probably do. The info SenorFunkyPants was provided by the Michelin rep kind of suggests that they would.

Where is the test that shaves a tire so much that there is not enough tread life left to even finish the session? I don't think that test ends up getting run very often and if so only by a big accident.

.... It also does sounds quite fishy that tread wear decreases but grip stays constant (again after about mid life). Lastly when does the plateau end does it simply hit a vertical "brick wall" right at the wear bar or is it right at cording? It must come down at some point and even get below the grip when just broken in. If it drops in the way he described then it must come down sharply, like going over a cliff.

There does seem a logical conflict between admitting that as you approach the wear bars there is a reduction in the tyre wear rate but no change in grip...suggesting that its a function of the tyres reduced ability to heat up I suppose is plausible , but not totally convincing.
I guess I could be a victim of "selective memory"...by coincidence the tyres on my M3s could have been wearing out as we entered into winter and the reduced traction was a function of reduced temperatures rather than an overall reduction in tyre grip.
As I noted before, a few years ago I had already discussed this apparent phenomenon with Michelin so convinced I was of its repeatability...now I'm begining to doubt myself.

As far as how grip changes as you pass the wear bars, we didn't really get into that. Here the weather in winter is such that running your tyres below the wear bars is not sensible and will get you into serious trouble if stopped by the police...each tyre below 1.6mm will get you 3 points on your licence - 12 points = lost licence.

Where is the test that shaves a tire so much that there is not enough tread life left to even finish the session? I don't think that test ends up getting run very often and if so only by a big accident.

Our fastest tire setup was always the new shaved to 2/32's(minimum tread depth by the rules) that we used for qualifying or a short race where the tires would still be quick and not be corded before the end of the race.If started on 4/32 with a few heat cycles on it became quicker as the tread disapeared with our fastest laps usually at race end ,but remember this is a single session with no further heat cycling.

Where is the test that shaves a tire so much that there is not enough tread life left to even finish the session? I don't think that test ends up getting run very often and if so only by a big accident.

Depends on what the test entails. You don't need a track session to test a tire's lateral grip on a skidpad or CoF. The Skip Barber people could have easily tested a tire they shaved to almost no tread in a relatively controlled environment (not necessarily on a car), and I don't see why that would be an accident. If you reread the quote from that book, you'll see that they are stating, in two consequent sentences, street tires are run from 5/32" down to bald and that there is no significant reduction in CF from 5/32" down based on their testing, so to me, that suggests a range for their testing and bald means almost no tread.

Regardless, that specific scenario is not relevant to this thread to begin with since, as far as I can tell, nobody here has referenced starting a track session or a race with tires that were about to cord. And, in the interest of safety, I hope nobody has plans to do so.

Our fastest tire setup was always the new shaved to 2/32's(minimum tread depth by the rules) that we used for qualifying or a short race where the tires would still be quick and not be corded before the end of the race.If started on 4/32 with a few heat cycles on it became quicker as the tread disapeared with our fastest laps usually at race end ,but remember this is a single session with no further heat cycling.

Even more interesting to hear they either retained their grip or possibly gained some grip as the tread wore down (in the absence of significant prior heat cycling as you point out). How much tread would you say you ended up with at the end of the short or the longer races? In other words, how close do you think the tires were to being corded?

Even more interesting to hear they either retained their grip or possibly gained some grip as the tread wore down (in the absence of significant prior heat cycling as you point out). How much tread would you say you ended up with at the end of the short or the longer races? In other words, how close do you think the tires were to being corded?

When the tread was almost like a holograph is where they were quickest and were only laps away from cording.They could have a fair bit of cording and not slow down as much as you would think they would.

When the tread was almost like a holograph is where they were quickest and were only laps away from cording.They could have a fair bit of cording and not slow down as much as you would think they would.

More interesting stuff; didn't know that you could get grip out of corded tires.

More interesting stuff; didn't know that you could get grip out of corded tires.

We are talking about Firerocks hereand we are talking about a very small percentage of the total tread area.dont forget that I run my 888's down to the cords and the only reason that I stop running them is that the treads start hitting the body

We are talking about Firerocks hereand we are talking about a very small percentage of the total thread area.dont forget that I run my 888's down to the cords and the only reason that I stop running them is that the treads start hitting the body

Another test I would love to do (as much as it would likely not even be rigorous and would only be a single data point) is to run back to back two sets of PS2, one say <10k mi, <6 months old, around 50% worn, vs. a new then immediately shaved set of PS2's, shaved just into the wear bar or say slightly above the wear bar. I think their is ample evidence and opinion here that they would be very close in traction or perhaps the latter having some advantage. I think the only way this could be the case is if those of us who have experienced significantly observable traction loss under a similar extreme wear scenario are also feeling significant negative/confounding heat/UV/aging effects.

As with many complex things I examine I simply like to see consistency among anecdote, testing, theory, common sense, etc. and this one is vexing...

Hmmm, essentially comparing a "half aged" tire versus a new shaved tire? I would have to think that the new shaved would always outperform, either by a little, or by a lot, depending on how aged the old tire is. But that's based on the conventional wisdom.

As an anecdote, I put a LOT of heat cycles on the PS2s on my GT3 when I first got it. After 25-30 track days, they had <10k miles, ~50% tread still left (wasn't driving the car hard yet), although they were more like ~1-1.5 yrs old and not <6 months. Car was garaged when not going to the track, so UV degradation shouldn't really be a factor. But I can tell you with that many heat cycles at that point in their lifecycle, they were hard as rocks, you could easily feel the difference in hardness when touching them. Those would obviously be expected to perform much worse than a new shaved set of tires, but this is an extreme. So maybe to tighten up the description of the older tire, maybe you would need to define it as worn down to 50% by "street" use without significant track heat cycles?

Conventional wisdom would say that the half aged tire should not outperform the new shaved tire. If not aged a lot, the older tire could perform close to the new shaved - but seems doubtful it would be better.

Oh, and by "new" I assume you mean broken in new, not literally "just came out of the mold and has never touched the road" new.